Etiket Arşivleri: FE 546

Blanching ( Dr. Manolya E. ÖNER )

•FE 546 THERMAL PROCESS ENGINEERING
•Manolya E. Oner, PhD
•Assistant Professor of Food Engineering
•FE 546 Thermal Process Engineering
•Food processing technology: Principles and practice. 2009. Third ed. P.J. Fellows
•Continuous thermal processing of foods: Pasteurization and UHT Sterilization. 2002. Michael Lewis and Neil Heppell
•Engineering aspects of thermal food processing. 2009. Ricardo Simpson.
•Thermal food processing: New Technologies and Quality Issues. 2006. Da-Wen Sun
•Heat Treatment
Heat treatment remains one of the most important methods used in food processing, not only because of the desirable effects on eating quality (e.g. baking), but also because of the preservative effect on foods by the destruction of enzymes, micro- organisms, insects and parasites.
•Advantages of Heat Processing
•Relatively simple control of processing conditions.
•Capability to produce shelf-stable foods that do not require refrigeration.
•Destruction of anti-nutritional factors (e.g. trypsin inhibitor in some legumes)
•Improvement in the availability of some nutrients (e.g. improved digestibility of proteins, gelatinization of starches and release of bound niacin).
•Blanching
Blanching serves a variety of functions, one of the main ones being to destroy enzymic activity in vegetables and some fruits, prior to further processing. As such, it is not intended as a sole method of preservation but as a pre-treatment which is normally carried out between the preparation of the raw material and later operations (particularly heat sterilization, dehydration and freezing).
•The Factors Influence Blanching Time
• Type of fruit or vegetable
• Size of the pieces of food
• Blanching temperature
• Method of heating
•Why blanching is necessary?
•The maximum processing temperature in freezing and dehydration is insufficient to
inactivate enzymes. If the food is not blanched, undesirable changes in sensory characteristics and nutritional properties take place during storage.
•In canning, the time taken to reach sterilizing temperatures, particularly in large cans, may be sufficient to allow enzyme activity to take place. It is therefore necessary to blanch foods prior to these preservation operations.
•Under-blanching may cause more damage to food than the absence of blanching does, because heat, which is sufficient to disrupt tissues and release enzymes, but not inactivate them, causes accelerated damage by mixing the enzymes and substrates. In addition, only some enzymes may be destroyed which causes increased activity of others and accelerated deterioration.
•Blanching reduces the numbers of contaminating micro-organisms on the surface
of foods and hence assists in subsequent preservation operations. This is particularly
important in heat sterilization, as the time and temperature of processing are designed to achieve a specified reduction in cell numbers.
•Blanching also softens vegetable tissues to facilitate filling into containers and removes air from intercellular spaces which increases the density of food and assists in the formation of a head-space vacuum in cans.
•Blanching Methods
•The two most widespread commercial methods of blanching involve passing food through an atmosphere of saturated steam, steam blanching, or a bath of hot water, water blanching. Both types of equipment are relatively simple and inexpensive.
•WATER BLANCHING
•Advantages of Water Blanchers
•Lower capital cost and better energy efficiency than steam blanchers.
•Limitations for Water Blanchers
•Higher costs in purchase of water and charges for treatment of large volumes of dilute effluent. Risk of contamination by thermophilic bacteria.
•STEAM BLANCHING
•A product is transported by a chain or belt conveyor through a chamber where ‘‘food-grade’’ steam at approximately 100 C is directly injected.
•Usually temperature in the headspace is measured and the flow rate of steam is controlled.
•Advantages of Steam Blanchers
•Smaller loss of water-soluble components.
•Smaller volumes of waste and lower disposal charges than water blanchers, particularly with air cooling instead of water.
•Easy to clean and sterilize.
•Limitations for Steam Blanchers
•Limited cleaning of the food so washers also required.
•Uneven blanching if the food is piled too high on the conveyor.
•Some loss of mass in the food.
•Individual Quick Blanching (IBQ)
•IBQ was developed to overcome with problems in conventional steam blanching:
– There is often poor uniformity of heating in the multiple layers of food.
-The time–temperature combination required to ensure enzyme inactivation at the centre of the bed results in overheating of food at the edges and a consequent loss of texture and other sensory characteristics.
First stage: The food is heated in a single layer to a sufficiently high temperature to inactivate enzymes. Second stage (termed adiabatic holding): a deep bed of food is held for sufficient time to allow the temperature at the centre of each piece to increase to that needed for enzyme inactivation.
•Effect of blanching on cell tissues:
•Effect of blanching on nutrients
Some minerals, water-soluble vitamins and other water-soluble components are lost during blanching. Losses of vitamins are mostly due to leaching, thermal destruction and, to a lesser extent, oxidation.
• the maturity of the food and variety
• methods used in preparation of the food, particularly the extent of cutting, slicing or dicing
•the surface-area-to-volume ratio of the pieces of food method of blanching time and temperature of blanching (lower vitamin losses at higher temperatures for shorter times)
• the method of cooling
• the ratio of water to food (in both water blanching and cooling).
•Effect of blanching method on ascorbic acid losses in selected vegetables
•Effect of blanching on color and texture
• The time and temperature of blanching also influence the change in food pigments according to their D value.
•The time and temperature conditions needed to achieve enzyme inactivation cause an excessive loss of texture in some types of food. Calcium chloride (1–2%) is therefore added to blancher water to form insoluble calcium pectate complexes and thus to maintain firmness in the tissues.

Heat Transfer by Convection and Radiation

FE 546 Thermal Process Engineering

Heat Transfer
Heat Transfer by Convection and Radiation
Heat Transfer by Convection
Convective heat transfer inside containers results either from the natural effects of changes in density in the liquid induced by changes in temperature at the container walls (free or natural convection) or by creating motion in the container contents by axial or end-over-end rotation (forced convection).

Mechanism of Natural Convection

The process of natural convection initially involves heat transferred by conduction into the outer layers of fluid adjacent to the heated wall; this results in a decrease in the density, and the heated fluid layer rises.
When it reaches the top of the liquid at the headspace, the induced fluid motion causes it to fall in the central core, the displaced hot fluid at the wall being replaced by colder fluid in the core.
As the temperature of the can contents becomes more uniform and the driving force smaller, the fluid velocity tends to decrease and eventually, when the fluid becomes uniformly heated, the motion ceases. Idealized representation of convection currents in a can of liquid
In the upper part of the container the hot liquid is being pumped by the heated fluid rising in the boundary layer, and being placed on the cold liquid in the core.
Simultaneously, in the case of a vertical container, there is also heat rising from the bottom end of the can, which produces mixing eddies in the bulk of the fluid.
Datta (1985) has shown that as result of instabilities in the temperature distribution, regular bursts of hot liquid occur on the base of the container. This phenomenon is known as Bernard convection.
The mechanism of unsteady-state convection is very complex and varies with time of heating and/or cooling; consequently it is very difficult to model precisely.
Hiddink (1975) used a flow visualizing technique with metallic powders in liquids in containers with light- transmitting walls, to highlight the streamlines in the bulk of the fluid.
Blaisdell (1963), have used thermocouples at several points in the containers to plot the temperature profiles.
Guerrei (1993) has discussed the internal flow patterns in convection-heating bottles of cylindrical and square shape, using the concept of a double-layer system of rising hot liquid on the wall of the container which discharge into a central volume. The point-of- slowest-heating was shown to be 0.006 m from the wall of an 0.06 m internal diameter container.
The most important point, from a practical safety point of view, is where the slowest heating point is situated.
While the discussion has been concerned with convection heating/cooling inside containers, there is also the problem of convection heating on the outside, from the heating or cooling medium to the container wall.
In the case of pure steam heating, condensation of steam on the container wall surface raises the temperature of the surface almost immediately to that of the steam and, consequently, no problem arises.
If, however, the steam contains air, either as an adulterant or intentionally, then the velocity of the mixture over the surface will affect the temperature of the surface and a more complex situation will arise.
Similarly, if water is used for heating, and also for cooling, then the wall temperature will be affected by the velocity of the heat transfer medium.
In all cases, except for condensing pure steam, it is necessary to consider external convective heat transfer to the outer surface of the container.

Basic Concepts in Convection Heat Transfer

Mathematical models for the prediction of temperatures in the heating of canned foods by convection are necessary in order to determine the process requirements.
There are three approaches to convective heat transfer:
1) the film theory
2) the use of dimensionless numbers
3) the more rigorous mathematical treatment of the basic fluid dynamic and heat transfer models.

1) Film Theory

The idealized temperature profile for a material being heated by a fluid and separated by a container wall Thus if T1 is the temperature of the heating medium and T4 is the temperature in the bulk of the fluid being heated, and the average film temperatures are T2 and T3, then an overall heat-transfer coefficient U (alternatively denoted by H) can be defined as follows:

2) Correlations for Predicting Heat-Transfer Coefficients

Engineering practice makes use of dimensionless numbers for correlating heat transfer coefficients with the physical circumstances of heat transfer and the physical properties and flow conditions of the fluids involved.
The four dimensionless numbers used in heat transfer studies are:
For forced convection; the correlation is

3) Models for Convection Heat Transfer

Some of the models that have been used to predict temperature distributions and velocity profiles in heated and cooled can liquid products.
The models may be classified as follows:
²Energy Balance Model
This is the simplest of the models.
By equating the overall rate of heat transfer into the
can with the rate of accumulation of heat inside the can, an energy balance equation can be established.
This equation shows that the rate of heating is an exponential function, which depends on the overall heat-transfer coefficient U (or the internal heat-transfer coefficient hint when steam is used as the heating medium with metallic cans), the surface area A, the mass of the contents and their specific heat, as well as the temperature of the heating medium and the initial temperature of the contents T0.
²Effective Thermal Diffusivity Model
This model makes use of the unsteady state
conduction model solutions and an apparent or effective thermal diffusivity.
This depended on the ratio of solids to liquid in the
container.
²Transport Equation Model
This is the most rigorous approach to determining the temperature distributions and the velocity profiles in containers filled with liquids.
The form of the equations depends on the geometry of the container; care should be taken to use the most appropriate coordinate system.
The equations which have to be solved in relation to the container boundaries are the equation of continuity:

Radiation Heating

Thermal radiation has a wavelength from 0.8 to 400μm and requires no medium to transmit its energy.
The transfer of energy by radiation involves three processes:
1)the conversion of the thermal energy of a hot source
into electromagnetic waves
2) the passage of the waves from the hot source to the cold receiver
3) the absorption and reconversion of the electromagnetic waves into thermal energy.
The quantity of energy radiated from the surface per unit time is the emissive power E of the surface.
For a perfect radiator, known as a black body, the emissive power is proportional to the fourth power of the temperature T :
The main application for radiation theory is in
relation to can sterilization using gaseous flames. The cooling of cans to the atmosphere also involves loss of heat by radiation as well as convection.
A more complex heat-transfer model for canned foods heated in a flame sterilizer was developed.
6. Radiation exchange only occurs between the flame and the can, and from the can to the surroundings.
7. The temperature of the flame is the adiabatic temperature calculated on the dissociation of the combustion species.
8. The dissociated species do not combine on the can surface.

Mechanisms of Heat Transfer

FE 546 Thermal Process Engineering

Heat Transfer

CONDUCTION

Mechanisms of Heat Transfer
There are three modes of heat transfer:
Conduction is the transfer of heat by molecular motion in solid bodies.
Convection is the transfer of heat by fluid flow, created by density differences and buoyancy effects, in fluid products.
Radiation is the transfer of electromagnetic energy between two bodies at different temperatures.
Heat Transfer by Conduction
Energy transfer by conduction takes place when different parts of a solid body are at
different temperatures. Energy flow in the form of heat takes place from the hotter, more energetic state, to the colder, less energetic state.
The quantity of heat transferred under steady-state conditions:
Formulation of Problems Involving Conduction Heat Transfer
The mathematical basis of the problems encountered in the determination of the temperature distribution in heating canned foods by conduction.
Unsteady-state conduction heat transfer equations is Fourier’s equation
A simpler equation is generally used
1. The surface temperature is prescribed and does not vary with time, i.e. a surface is exposed to an instantaneous change in temperature. It applies to steam heating and is often assumed in heat transfer modeling work.
The average temperature will be signified by putting a bar over the temperature term, thus the volume-average temperature is given by